Thanks to everyone for working with us to identify and resolve this issue.

We have some automated security measures in place that prevent access from IP addresses that are seen as abusive or possibly harmful to WordPress.com. Sky, the ISP who was blocked, routes traffic to the USA (and possibly other countries) for many of their customers over a small number of IP addresses, so in this case, one bad actor on Sky's network caused multiple Sky customers to get blocked from accessing WordPress.com. We have identified the IP addresses in question and worked around this issue for now.